UPDATED 7:03 p.m ET
A handful of lawmakers, including House Speaker John Boehner, have received threatening letters in the last few days with an unidentified white powdery substance and more may be on the way, law enforcement officials told CBS News Wednesday.
In addition Boehner, at least one lawmaker from Wisconsin and a senator from Indiana have received the letters at their district offices, with more expected to receive additional letters, the officials said.
"Those letters (already received) were tested and the substance found to be harmless," said Terrance Gainer, Senate Sergeant at Arms.
In a letter to the Senate community, Gainer, the chamber's chief law enforcement official, said the author of the letters suggested some of these letters yet to arrive at Senate offices "may contain an actual harmful material."
"Although all letters received thus far have proved harmless, it is essential that we treat every piece of suspicious mail as if it may, in fact, be harmful," Gainer wrote in his memo to the Senate community.
The letters make vague complaints about too much money in politics and had a Portland, Oregon return address from an organization listed as "The MIB, LLC," a law enforcement official told CBS News.
In addition to the letters to the lawmakers, officials said television comedians Jon Stewart and Stephen Colbert received letters mentioning the letters to senators.
The author told the comedians he would send letters to all 100 senators and ten percent of them would contain "lethal pathogens," an official told CBS News.
The author wanted an end to corporate money and lobbying, an end to "corporate personhood," and called for a new constitutional convention to rewrite the constitution.
The author also told the comedians he would tell the senators they are "working for the wrong side" and there is a ten percent chance they have been exposed to a lethal pathogen. The author also said he "randomized" which letters would contain the pathogen and even he did not know who would get which letter.
The letters come more than a decade after five people, including two Washington postal workers, were killed when anthrax was mailed to the Washington offices of a pair of Democratic senators and a number of media personalities. The mail system to the U.S. Capitol was completely overhauled in the wake of those attacks and letters are now sent through and off-site processing facility before arriving at lawmakers' offices.
Additional reporting by Bob Orr, Nancy Cordes, Rob Hendin and Jenna Gibson.